This invention relates to charge pump circuitry. More specifically, it relates to charge pump circuitry and methods that provide reduced input and output current variation.
A charge pump is an electronic circuit that typically uses one or more capacitors as an energy storage element to create either a higher or lower voltage from a power source. Charge pump circuits generally operate by controlling the connection of a voltage source to one or more capacitors, usually in two or more phases. For example, in a simple charge pump circuit, to generate a higher voltage, a positive terminal of a capacitor is connected to the voltage source during a first phase. During this phase, a charge is imparted to the capacitor. Next, during a second phase, the capacitor is disconnected from the voltage source and reconnected with its negative terminal coupled to the voltage source.
Because the capacitor retains the difference in voltage between the top and bottom plates, the value of the voltage at the positive terminal is double that of the charging voltage. When the capacitor is discharged, usually into a filter capacitor, an output voltage up to twice the value of the input voltage is provided. Using this principle, further voltage multiplication may be achieved by adding additional capacitors in series to increase the multiplication factor. Variations of this principle allow circuit designers to also obtain fractional voltage multiplication factors, such as two thirds, or one and a half, through the use of the appropriate circuit topology. The frequency with which the charge pump changes phases is typically within the kilohertz to megahertz range and may be selected to minimize the size of the required filter capacitor.
Conventional charge pump circuits, however, exhibit undesirable current characteristics during switching intervals. For example, at the moment switches first connect the charge pump capacitors to a voltage source, a large inrush current flows, charging the capacitors. If the equivalent input impedance of the charge pump is small, this inrush current will be initially very large, then quickly decay, which produces a spike or “impulse” load on the input voltage at the switch transition point.
Because these switch transitions occur at a relatively high frequency, certain sensitive systems will be disturbed by this behavior, particularly if the source impedances in the system are comparable to the switch and wiring resistance of the charge pump circuit. Moreover, these impulses may contain substantial high-frequency content that can excite any high-Q elements they couple to, e.g. parasitic inductance and bypass capacitances. Similarly, the output signal of the charge pump circuit also undesirably includes such spikes at the switch transition point due to the discharge and subsequent decay of output current from the charge pump capacitors.
Various solutions have been proposed to address this problem, some of which have involved forcing a substantially constant input current to the charge pump. This may involve, for example, the use of two or more charge pump systems, each operating in alternative modes (one discharging while the other is charging, etc.). Nevertheless, such systems still produce spikes on their output during switch transitions because of the need for asymmetrical switching to properly operate, which results in the production of either too little or too much current at certain transition points.
Accordingly, in view of the foregoing, it would be desirable to provide circuitry and methods for charge pump circuitry that provide improved current characteristics.
It would be additionally desirable to provide circuitry and methods for charge pump circuitry that reduce or substantially eliminate input and output current variation.
It would be additionally desirable to provide circuitry and methods for charge pump circuitry that produce a substantially constant input and output current over a switching cycle.